To do this, open your Gmail account, scroll to the bottom of the page and click on the Details link in the bottom right of your screen, below last account activity.

This will open up a pop up window where you can see recent activity and – crucially – whether your Gmail or Google Apps account is open in any other location. The list also shows the location of any authorised apps.
If you want to sign out of any open sessions, click the Sign out of all other web sessions button. This will ensure all your previous Gmail and Google Apps sessions are logged out, leaving you logged in to the current computer only.